1. Field of the Invention
The present invention relates to an electronic engine ignition control device for automatically controlling an engine ignition to an optimum state in response to an engine speed.
2. Description of the Prior Art
Heretofore, in order to effect an adjustment of a spark-advance angle of an engine at its ignition time and an ON-OFF duty control of a switching power transistor arranged in a primary side of an ignition coil in response to an engine speed, an engine ignition control device of two-pulse system, as shown in FIG. 7, has been proposed, which includes means for generating two reference pulse signals Pa and Pb which are synchronized with the rotation of the engine and which vary their phase difference in response to the engine speed, means for generating a first signal Sa formed by producing a triangular wave signal at the timing of said reference pulse signal Pa and holding its peak value, means for generating a second signal Sb formed by producing a triangular wave signal at the timing of said reference pulse signal Sb and holding its peak value, and means for comparing one of the signals Sa or Sb with the other one of the signals Sb or Sa which has been added with a pretermined bias and clamped, thereby obtaining crossing points a and b of the peak value of one of the triangular wave signals and a leading edge portion of the other trianglar wave signal, said points a and b forming OFF and ON time points of said power transistor, respectively. (See Japanese patent publication No. 57631/1983 and No. 60987/1986).
In the conventional engine ignition control device of two pulse system, as described above, a level of the signal which holds the peak value of the triangular wave signal varies in accordance with variation of the engine speed, but in the case of the leading edge portion of the triangular wave signal which does not hold its level owing to its characteristics, it cannot be used as a threshold value to be compared with the other triangular wave signal. Accordingly, the duty control range as well as the spark-advance angle range are narrowered and even if a satisfactory ignition control can be attained in case of an engine of a two-wheel vehicle, a satisfactory ignition characteristic cannot be attained in case of a four-wheel vehicle or the like where duty control is required to obtain delicate spark-advance control and high ignition energy enough to meet the regulation of exhaust gas. In FIG. 4, T1 indicates a duty control range in one ignition cycle T and T2 indicates a speak-advance range in the one ignition cycle.
In case of the conventional engine ignition control device of two pulse system as described above, a particular spark-advance characteristics which depends upon a phase difference .theta. between the two reference pulse signals Pa and Pb, as shown is FIG. 8, only makes a linear change within an engine speed range .DELTA.N determined by circuit constants, so that it is impossible to freely obtain an optimum spark-advance characteristic at the time of ignition as required by the engine.